1. Field of the Invention
The present invention relates to a rotation angle detection device that outputs a sine wave signal or a cosine wave signal relative to an angle, and an electric power steering apparatus that employs this detection device.
2. Description of the Background Art
To exercise position control or speed control by employing a motor, the rotational position of the shaft of the motor is detected. A method to detect the rotational aspect is a rotary encoder or a potentiometer. In particular, a non-contact potentiometer that employs the magneto-resistive effect of a semiconductor and outputs a sine wave signal is used as a sensor (see Robot Sensor Guide, by Eiji Koyanagi, Ohm Co., Ltd., Apr. 20, 2004, p. 58).
An electric power steering apparatus is an apparatus that transmits to a steering system an auxiliary torque generated by a motor, and reduces steering-wheel torque. The rotation angle of the motor shaft is detected by employing a resolver, which controls the motor. In Japanese Patent Unexamined Publication No. JP-A-2000-39336 (claims 1 and 2, and paragraph 0005), an example resolver is disclosed wherein a secondary winding is coupled with a primary winding through which a sinusoidal current is supplied, and a change in the rotation angle is output as an angle modulation signal of the sinusoidal current. According to this technique, an operation for determining whether a resolver output abnormality has occurred is performed based on the symmetry between the top and the bottom of the amplitude of the sine wave that is output. Further, another method is also disclosed whereby a difference between the maximum value and the minimum value of the sine wave that is output is employed to detect a sticking fault, which is when the output of the sensor is fixed to a specific value. In addition, a method is also described for calculating the sum of the squares of a sine wave signal and a cosine wave signal to detect a disconnection (see Robot Sensor Guide, cited above).
However, according to the technique in JP-A-2000-39336, whereby an operation for determining the occurrence of an abnormality is performed by using the values of a sine wave signal and a cosine wave signal, noise would be superimposed on the signals. Thus, a predetermined detection margin is required, and an abnormality can not be detected unless the abnormal state continues for an extended period of time.
Further, when the detection method is used to calculate the sum of the squares, a reliable detection process can not be performed when the values of the sine wave signal and the cosine wave signal are not greatly changed.
In addition, since the values of the sine wave signal and the cosine wave signal may be “0”, a predetermined elapsed period of time is also required in order to determine whether the current state is a sticking fault wherein the output is fixed to a specific value.